Christopher Paolini, the 28-year-old author of the bestselling Inheritance series, still lives at home with his parents and his younger sister. The family residence, a swanky cabin, is located in a remote corner of southwestern Montana known as Paradise Valley. It's an enormous, austere home, with plenty of private space for everyone – especially Christopher, who has his own wing.

"This is the lair," Paolini announces proudly, as we enter the two-room suite where he sleeps and works. "Nice place to write, huh?" he says, with a sweep of his hand, palm open, like a game-show host highlighting prizes.

The rooms are scrupulously neat and filled with dragons: There is a dragon coffee table, a dragon side table, a dragon sconce, a dragon shelf, a dragon clock, a dragon mirror, a dragon lamp, a dragon letter holder, a dragon bench and a dragon head mounted outside the door – a fitting design scheme for an author who made his name writing books about a boy named Eragon, who seals his fate as a "Dragon Rider" when he finds a bright-blue egg that hatches a dragon. "I could have gotten more," Paolini says, "but I decided that was enough dragons for one place."

There are also swords, lots of swords. "I want to show you this," he says, lifting up an impressive three-and-a-half-foot broadsword in a black leather sheath. "I bought this as a treat for finishing the book," he says, referring to Inheritance, the fourth and final installment in his epic cycle. The novels recount the adventures of Eragon and his dragon companion, Saphira, with whom he shares a telepathic connection, as they zoom around consorting with elves and dwarves and battling various dark forces dispatched by the evil King Galbatorix. ("I write schmaltz with dragons," says Paolini about his work.) Inheritance, which was published in November, sold 489,500 copies its first day out – more than any other release in 2011.

Impressive as that statistic sounds, it's akin to just another billion burgers on a McDonald's sign. Eragon, Paolini's first work of fantasy, published when he was only 19 years old, debuted at number three on the New York Times bestseller list, then spent 21 weeks at number one. His four books have collectively sold more than 33 million copies worldwide. Given Paolini's geographic isolation, the rudimentary nature of his plots and, of course, his startling youth, this is a powerfully bizarre achievement. But it is not his alone. About a decade ago, the Paolinis made the rather unconventional decision to make their teenage son's fantasy novel the family business. Paolini repeatedly emphasizes that he's spearheading a team effort, with Mom fielding fan mail, Dad turning "the financial wheels," sister acting as girl Friday, and all four editing. Together they have forged a prodigious empire: the Inheritance series is a multimillion-dollar mom and pop – and brother and sister – business run by a clan of introverts living on the prairie. "If it weren't for the support of my parents and sister," Paolini says, "none of this would've been possible for me."

Paolini wears small, round, wire-rimmed spectacles that have earned him endless comparisons to Harry Potter. His dark-brown hair, meticulously combed into place, appears never to move. He has his own tidy uniform of sorts: an ironed button-down shirt securely tucked into belted dark-wash jeans. He's endearingly nerdy – not in the affected, pretentious, nouveau-hipster sense, à la Wes Anderson, but rather in the traditional, unintentional, pale-as-your-inner-thigh sense, à la the kid who plays Dungeons & Dragons or Magic: The Gathering.

"Every fantasy author deserves a good sword, and this is mine," Paolini says. Slowly, dramatically, he removes the weapon from its scabbard. It makes a loud scraping noise on its way out. "This is the real deal," he says, waving the blade, made of high-carbon Damascus steel and etched with an elaborate calligraphic design, in front of him. "I found the only custom sword store in all of New York City," he continues, slicing the air. "It fits me and my body perfectly. I hope never to have a duel, but if I do, I would trust my life to this sword."

"Please remove your shoes," says Paolini's 26-year-old sister, Angela, with dramatic singsong inflection, as she meets me in a foyer that doubles as a home gym. Paolini's mother, Talita, a birdlike woman with a girlish voice and a sweet manner, makes a fluttery motion with her hand to indicate, "Oh, forget it, she can leave them on." But Angela, who is tough and no-nonsense, and acts as her brother's protector, insists: "We don't wear them in the house."

According to Paolini, he and Angela, a petite woman with intelligent dark eyes and a fantastic nest of dark, springy curls, have their own near-telepathic bond, not unlike Eragon and his dragon. Angela is the inspiration for one of Paolini's fictional characters, the quirky, feisty Angela the Herbalist, who plays a pivotal narrative role by telling Eragon his fortune, and is the most captivating character in the books. In one passage from Inheritance, Paolini writes that Angela the Herbalist's expression is "languid and insolent"; the description applies to the real Angela as well. She gives the discomfiting impression that she's mocking you, teasing you, flirting with you and sizing you up all at once.

Paolini and the female half of his family gather, barefoot, in an airy main room with high, slanted ceilings. We all settle into a quartet of ergonomic leather chairs. Paolini's father, Kenneth – olive-skinned, compact, a human Tesla coil of intensity – is preparing to grill some hormone-free burgers. Picture windows offer a view of dry, maize-colored grass rippling in the wind, and, in the distance, the blue-tinged Absaroka mountains. A set of djembe drums rests against one wall. "Who plays the drums?" I ask. "Oh, we all do," Paolini says with a shrug, as though this is the most natural thing in the world.

Later, while we eat, Paolini tells me that every evening during dinner the family watches a movie together. The night before my visit, it was the Fellini film Il Bidone. "Tonight we'll probably watch Transformers: Dark Side of the Moon," Paolini says. "We watch anything we want to."

If you know something about where Christopher Paolini lives, and something about his family, his novels become less fantastical than they might at first seem. "People think I'm inventing a lot of the stuff, and I do," he says, "but a lot of it is based on personal experience." The books are set in a sort of medievalized version of Montana, as strange and psychedelic a natural environment as exists anywhere. In Yellowstone Park, a few miles away from his home, there are thermal geysers from which steaming water thunders out of the earth, sulfurous green pools, bubbling mud pots and colorful bacterial mats. Wall-shuddering gales blow for weeks at a time and are especially pronounced in Paradise Valley. The intensity of the elements creates a timeless atmosphere, one very much like "the harsh, beautiful land" of Alagaësia, the imaginary world of the series. As Paolini puts it, "Most of my descriptions of nature, the way ice looks, or the way the trees look – it's all from here."

Paolini has lived in Montana since he was three. His parents moved here as members of the Church Universal and Triumphant, also known as CUT, a New Age survivalist cult. CUT was as steeped in faux-medieval symbolism – gnomes, sylphs and fairies – as any fantasy book or video game. The group's leader, Elizabeth Clare Prophet, or "Guru Ma," channeled the mystical teachings of the "Ascended Masters," a pantheon of enlightened beings ranging from Jesus to Maria Montessori. One of CUT's missions was to revive the quest for the Holy Grail; Prophet imagined herself as Queen Guinevere, and other church figures as Lancelot, King Arthur and Sir Galahad. In the mid-Eighties, Prophet began warning of an impending apocalypse. She ordered her followers to leave "Camelot," the group's headquarters in the Santa Monica Mountains, and relocate to Montana, where members were building underground bomb shelters. The Paolinis joined the exodus but soon grew disenchanted with the group, leaving in 1987 because they refused, in their words, "to sacrifice our family to CUT and its mission."

Paolini downplays his parents' decade-long membership in CUT, saying that it didn't really affect him, because they left when he was just a boy. This seems unlikely, given that CUT haunted his family for years. In 1989, when Prophet's husband and another member of the group were arrested for buying firearms illegally, Kenneth gave an incriminating interview to the local paper, saying that Prophet and her henchmen had been stockpiling weapons. The story blew up. The Paolinis did scores of interviews, and Kenneth appeared on Oprah. In 1999, when Paolini was 15 years old, his parents self-published an account of their CUT experience, 400 Years of Imaginary Friends. "We discovered that CUT was the culmination of centuries of hoaxes," they write, "that it was built on myth and sold as reality."

The years Paolini's parents spent immersed in CUT's Led Zeppelin theology seem to have penetrated their son's consciousness. One might even argue that his fascination with mythical beings simply took a more socially acceptable form. But perhaps the most obvious effect on his writing is an aversion to groupthink. One of the major preoccupations of the Inheritance series is that Eragon remain independent of mind, free of allegiance to any authority, answering to his conscience alone. "I definitely have an appreciation of how easy it is for people to end up getting led astray if you let other people make decisions for you," he says, "and part of that certainly comes from family experience."

On a bright fall afternoon, I meet Paolini for lunch at a bar in Livingston, Montana. He has been in the public eye for almost a decade now, and interviewing him can be an opaque affair. He lights up when the topic is fantasy literature; ask him about almost anything else, though, and he grows visibly bored and distracted. Inquire about his personal life – his religious beliefs, or whether he has a significant other – and the curtain comes down. "I'd rather not say," he'll answer, like a seasoned politician.

But he loves telling his own origin myth. He led an isolated childhood. Even after leaving CUT, his parents remained apart from mainstream life. Kenneth earned a modest living doing odd jobs like Rolfing (a form of massage therapy). For a time, the family lived in an airplane hangar, then in a cabin prone to leaks and heated by a 55-gallon barrel stove. Both children were home-schooled. Christopher devoured all the fantasy books at the tiny Livingston library, until he felt "as if I'd read all the fantasy stories out there." Restless, curious and free of the confines of conventional schooling, he became the sort of kid who teaches himself to make chain mail. "I built two forges when I was in my teens," he says. "I was just really, really into metalworking and making stuff."

When he graduated at 15 from the American School, an accredited distance-learning institution, his parents felt he was too young to go to college. He didn't have a driver's license, a job or much to do. He began digging a crater in his backyard – "I decided I wanted to make an underground lair," he says. "You know, like a Hobbit hole" – but in two weeks, he'd tunneled as far as the arid Montana earth would allow, and he was in need of a new diversion. Driven by typical adolescent yearning, at once vague and grandiose, he decided to write a book. "I was reading about Renaissance men and women in their early twenties," he says, "and they'd done all this incredible stuff – spoke four languages, played the violin. So I was looking for something big to do."

He decided to plot a trilogy because "all fantasy stories are trilogies, right?" A year later, having completed a draft of the first book, he couldn't let it go. "I'm the sort of person that starts digging a hole and doesn't stop until it's finished," he says. He took a second year and revised it. He fleshed out characters, rewrote dialogue, changed much of the prose. Then he gave it to his parents, who by that time had a small self-publishing company they called Paolini International. "They thought it needed editing," he says, "but they liked it."

Paolini deferred college (he'd been accepted to Reed), and his parents went to work on Eragon. He peddled his novel at local shops dressed in medieval costume: a billowy red swordsman shirt, black pantaloons, black knee-high boots, black pirate sash and a black beret. "I would talk to every person who came into the store for eight hours straight," he says. He became the family breadwinner. "I was in the situation where books sold meant food on the table," he says. "A lot of authors are, but most aren't physically selling every single book." He could sell 40 books a day – not enough. The family's finances dwindled.

As luck would have it, the writer Carl Hiaasen came to vacation with his family in Livingston. His wife bought a copy of the self-published Eragon at the supermarket for their preteen son, Ryan. "I remember driving around and not a peep coming from the back seat of the car," Hiaasen says, "and Ryan just blazing through the book. Finally I said, 'How do you like it?' He looked up and said, 'Dad, this is better than Harry Potter.'" Hiaasen sent the book to his editor, and Knopf ended up buying Eragon, as well as two additional books from the unwritten trilogy (the overlong third was later split into a fourth). "I've been under deadline ever since," Paolini says.

Paolini and I are back in his boy cave, the one place he seems completely at ease. Hanging out in this room filled with homemade chain mail, sketches he's penned, a diary he's encrypted "in runes," and all those daggers and dragons, is like inhabiting his imagination. You can't help but wonder about the next thing to spring from it. Paolini hints that he might try science fiction, but says, "Don't hold me to it." He wants to take a little time off. The writing of Inheritance was excruciating for him. He was mainlining coffee and chocolate, grinding out only 350 words a day. It turns out he was suffering from an undiagnosed thyroid disorder, and he still seems drained from the ordeal.

After all, Paolini had three other people depending on him. Now that he's done, there's a different kind of pressure: The lives of his loved ones will change course as his own life does. Usually a rite of passage affects one person, not four. "We're going to be working in a different manner in the future," he says. "We'll still work together. I mean, you're always stronger together than you are apart."

The major criticism of Paolini's work is that he has recast timeworn fantasy elements and themes in an unoriginal way. To the haters, he's a plagiarist who has blatantly lifted ideas: plot from Star Wars, rules of magic from Ursula K. Le Guin, dragon riders from Anne McCaffrey. "Shall we ding Joyce for reworking The Odyssey?" Paolini says. "That's the way culture works."

In the acknowledgments at the end of Eldest, his second novel, Paolini writes, "Eragon's journey has been my own: plucked from a sheltered rural upbringing... enduring intense and arduous training; achieving success against all expectations; dealing with the consequences of fame; and eventually finding a measure of peace."

Obvious as the parallel is, melodramatic as the writing is, with this paragraph, Paolini pinpoints the ultimate appeal of the series. The books read as authentic because Paolini himself has grown up over the arc of the story, as Eragon has. Both boys were 15 when their journey began, and are adults when it ends.

Will Paolini ever leave home, as Eragon eventually does, now that his quest is over? "I sort of put a lot of things off because they would've been too disruptive to handle along with finishing the series," he says. I ask him what "things" he's referring to. For a rare moment, Paolini lets his guard down. It's like a sliver of light passing over a bedroom wall as a car drives by at night – perceptible, then gone. "Well, a lot of people move out of the house when they go to college," he says. "I certainly don't mind working here... It's the ivory tower every author could wish for. But it's refreshing to not have to worry about a deadline."